Pain from degenerative spine disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. The traditional surgical treatment of a degenerating and painful inter-vertebral disc has been the complete immobilization and bony fusion of the involved spinal segment. An extensive array of surgical techniques and implantable devices have been formulated to accomplish this goal.
The growing experience with spinal fusion has shed light on the long-term consequences of vertebral immobilization. It is now accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to the adjacent, degenerating levels. The second procedure necessitates re-dissection through the prior operative field and carries significantly greater risk than the initial procedure while providing a reduced probability of pain relief. Further, extension of the fusion will increase the load on the motion segments that now lie at either end of the fusion construct and will accelerate the rate of degeneration at those levels. Thus, spinal fusion begets additional fusion surgery.
There is a growing recognition that segmental spinal fusion and complete immobilization is an inadequate solution to degenerative disc disease. Replacement of the degenerated and painful disc with a mobile prosthesis is a more intuitive and rational treatment option. This approach would permit preservation of spinal mobility in many patients with degenerative disc disease. Eventually, the degenerative process will progress sufficiently so that motion preservation with a mobile prosthesis is no longer possible. Those patients may be treated with fusion. That is, fusion and complete segmental immobilization is reserved for those patients with advanced degenerative disease where the spinal segment is beyond surgical reconstruction.
U.S. Pat. Nos. 4,759,769; 4,997,432; 5,674,294; 5,674,296; 5,676,701; 5,888,226; 6,001,130; 6,019,792; 6,162,252; 6,348,071; 6,368,350; 6,419,706; 6,520,996; 6,540,785; 6,607,558; 6,645,249; 6,673,113; 6,749,635 and many others have illustrated various artificial disc prosthesis. Despite the large number of proposed designs, several issues remain poorly addressed.
The cervical and lumbar spinal regions experience the greatest amount of degeneration and will be the most common recipients of artificial disc devices. However, the movement characteristics at these two anatomical regions are different. In the sagittal plane of cervical and lumbar spines, the motion of the upper vertebra onto the lower vertebra forms an actuate pathway with a center of rotation below the upper surface of the lower vertebra. Further, in the coronal plane of the lumbar spine, the motion of the upper vertebra onto the lower vertebra also forms an actuate pathway with a center of rotation below the upper surface of the lower vertebra. However, in the coronal plane of the cervical spine, the motion of the upper vertebra onto the lower vertebra forms an actuate pathway with a center of rotation above the lower surface of the upper vertebra.
Each spinal motion segment is composed of two adjacent vertebras and the articulations between them. These articulations include the anteriorly positioned inter-vertebral disc and the two posteriorly positioned facet joints. In the transfer of vertical force between adjacent vertebral bodies, the inter-vertebral disc caries approximately 80% of the load while the remaining 20% is borne by the facet joints. A predominate function of the facet joints is to limit the extent of rotation and forward translation between the adjacent bones.
Since the articulation between adjacent vertebral bones is composed of the inter-vertebral disc and two facet joints, any attempt at restoration of vertebral motion must address all three components of the articulation. Replacement of the painful disc with an artificial prosthesis will restore a more full range of motion to the segment and those patients with extensive degenerative disease of the facet joints will experience an increase in facet joint pain after artificial disc implantation because of the increased motion. For this reason, artificial disc placement is contraindicated in those patients with significant facet joint disease. Similarly, those with healthy facet joints at the time of implantation will develop pain as these joints degenerate over time. In fact, the rate of facet joint degeneration and the subsequent development of pain are emerging as major determinates of the clinical success of artificial disc replacement. That is, patient who undergoes artificial disc replacement to treat back pain will have re-emergence of the pain symptoms as the facet joints degenerate and the rate of joint degeneration will determine the time until symptom recurrence. Since the useful life of the prosthesis greatly exceeds the life expectancy of the degenerating facet joint, the rate of joint degeneration becomes the true determinate of the pain-free interval that resides between the time of prosthesis implantation and the time of pain recurrence. The pain-free interval is a prominent statistic in the overall determination of clinical success of these operations.
The design of the implanted disc prosthesis can significantly influence the rate of facet joint degeneration. As expected, a disc prosthesis that significantly loads the facet joints will accelerate the rate of joint degeneration and shorten the pain-free interval. Biomechanical studies have shown that the stress forces inside the facet joints tend to be highest when rotational forces are applied to the motion segment. Prosthetic discs that increase the extent of rotational freedom necessarily increase the load on the facet joints. Further, extreme extension of the motion segment will cause the two joint surfaces to “bottom out” and forcefully abut one another which also increase the rate of joint degeneration. Finally, ball-in-socket device designs with a fixed center of rotation and a large ball radius can produce significant anterior translation of the upper vertebral body with flexion and cause pronounced loading of the facet joints. Conversely, devices with a small ball radius can produce significant facet joint abutment in flexion and an increase in the rate of joint degeneration. Hence, despite advances in implant design, there remains a need in the art for improved implants.